Damping of vocal fold oscillation at voice offset

Vocal folds show a damped oscillatory movement while abducting at the end of a vocal emission. The phenomenon can be observed with high-speed videoendoscopy and with different glottographic methods. It reflects important mechanical properties of the vocal oscillator, and cannot be voluntarily controlled. It could become a valuable clinical parameter, particularly in a medicolegal context, but its large variability in a same subject limits its use. First, possibilities and limitations of each recording method are reviewed. Second, the three main physiological factors accounting for the variability are analysed: (1) the timing dynamics of the expiratory pressure with respect to the opening of the glottis; (2) the speed at which vocal fold edges are abducted and glottal resistance drops, the combined effect of (1) and (2) determining the persisting transglottal flow, hence a persisting driving force; (3) the morphological change of the oscillator, whose lip-like shape becomes flattened depending on the degree of abduction. For clinical/medicolegal applications, additional research is required as to the recording protocol. A possible solution could be an entire recording with high speed transnasal videokymography of a standardised passage read by the subject, with a posteriori automatic extraction, by dedicated software, of all damping phases and computation of the average damping coefficient.